The present invention is directed to a method and apparatus for rejuvenating drilling mud from an oil, gas or other similar well.
In the drilling of oil wells, drilling mud is forced down the drill string emerging therefrom at the drill bit, and then passes upwardly on the outside of the drill string, between the drill string and the wall forming the bore hole. The drilling mud functions to carry the cuttings to the surface, to control the pressure in the formation being drilled to lubricate the drill string and the bit and to avoid sidewall collapse of the bore hole. At the present time, various additives are supplied to the drilling mud to improve its performance. Since the drilling mud has substantial value it is desirable that it be recovered, and rejuvenated for reuse. Rejuvenation includes the removal of solids, which are introduced into the drilling mud as cuttings. The terms "solids" and "cuttings" as used hereinafter refer to stone or earth particles cut from the earth by the drilling bit. The condition of the drilling mud is of extreme importance. It is known that the inclusion of solids in the drilling mud has such harmful effects as decreasing the life of pumps and other expensive equipment. Further, it is desirable that expensive additives, such as barite not be discarded, that substantially all of the entrained gases and air be removed, and that the drilling mud not be diluted. At the present time, it is conventional to remove solids by such devices as shale shakers, desanders, desilters, and centrifuges. Degassers are also utilized.
Thus, the drilling fluid must be maintained in good physical and chemical condition in order to be able to do its work. The cuttings range in size from large pieces (3/4 in. diam.) to very fine particles. The large cuttings, if permitted to be circulated in the fluid down the drillpipe, would plug up the rock bit channels and the drill string tools and must be removed from the drilling fluid (mud). Removal of the large cuttings is accomplished easily by passing the drilling fluid stream over a vibrating coarse screen (shale shaker) where the large pieces are screened out. On the other hand the smaller cuttings particles in the drilling fluid are not easily removed and their presence can have very detrimental effects on the entire drilling process. As solids content builds up, rate of penetration of the bit is reduced (due to unnecessary redrilling of rock), severe abrasion of the rock bit and drill string tools takes place and the properties of the drilling fluid itself are seriously impaired and tend to become uncontrollable.
One method of removing the smaller sized particles from the mud stream is by dilution of the whole mud volume by adding water. This is often impractical and almost always expensive. The costs of adding water, providing for more pit volume, etc., will generally make it more desirable to remove solids mechanically such as by screening. However, dilution is frequently used to reduce solids content during drilling of the surface hole (the upper or shallower portion of the hole).
In a typical apparatus, the drill mud from the drilling apparatus is passed through six modular units or work stations comprising: (1) a shale shaker; (2) a degasser; (3) a desander; (4) a desilter; (5) a mud cleaner and (6) a centrifuge. The mud is initially fed into a shale shaker which separates large particles with the remaining drilling mud then being delivered to a first tank. It is pumped from the first tank, through a degasser, and enters a second tank. It is then pumped from the second tank and passed through a desander, and enters a third tank. It is then pumped through a desilter, or a mud cleaner, which, like the desander, is in the form of a cyclone filter or cleaner. It then enters a fourth tank, and it is delivered to the mud tank or mud holding tank from the fourth tank.
Accordingly, the mud is moved in a batch type operation form one module (unit) of equipment to the next module by means of at least three energy consuming centrifugal pumps. Since the use of all modules of the equipment is not a continuous operation, only a portion of the total drilling fluids stream is actually processed through the entire system. The economics of a mud system depend heavily on the daily treatment costs while drilling. Mud is treated in two ways--with treating chemicals and with mechanical equipment. One of the primary causes of high daily treatment cost is due to cuttings (also referred to as "solids") which become incorporated in the mud. The solids are chopped into finer particles each time they pass through a pump; this causes substantial wear and tear on the pumps and also makes removal of the solids even more difficult due to the buildup of the small particles.
The solids consequently create many problems as they become dispersed throughout the mud system and since they accumulate with continuous drilling, they must eventually be dealt with. The solids are removed by batch type fine screening, desanding, desilting, and centrifuging operations. Except for screening, which classifies and removes drilled solids which are coarser than the openings in the screen, the other types of equipment employed are extremely inefficient in removing solids. In fact, hydrocyclones, which are used in desanding and desilting and centrifuges are not classification devices.
It is generally accepted in the industry that present equipment usages permit no better than 50% removal of the solids of given size from the mud. While some mud systems have multiple units for "better" solids control, the costs are excessive. On a typical deep well where mud costs are high, equipment rental costs on the average are $1,500 per day. Deeper wells sometimes incur equipment rental costs in excess of $2,000 per day. Horsepower requirements for the pump and module operation of the equipment ranges from 430 to 450 Hp. Moreover assembly and piping costs often equal the buy-out price of the equipment. It will consequently be apparent that present known approaches to drilling mud treatment are both functionally inadequate and financially burdensome.
Although there have been proposals for simpler methods and apparatus for treatment of drilling mud such as in Lee U.S. Pat. No. 4,350,591, such apparatus has not found any substantial acceptance in the industry. This apparatus utilizes a filter belt passing over drums, and vibrating plates in engagement with the drilling mud delivered to the filter to assist in forcing the drilling mud through the filter.
Consequently, the heretofore used equipment has been intended to solve the problem of removing solids with it being recognized that the solids are in the form of particles varying in size between about 1.5 microns up to about 0.75 inch. To properly rejuvenate drilling mud, these solids must be substantially entirely removed, and, in addition, gas contained in the drilling mud must be removed to the fullest extent possible.
Therefore, the primary object of the present invention is to provide new and improved methods and apparatus for the treatment of drilling mud.